A program's complexity can increase very quickly as the program is developed or updated. It often becomes increasingly difficult to detect deficiencies such as logic errors, security vulnerabilities, overlooked input possibilities, and even some typographical errors in the program or its underlying source code. Some errors, such as some misspelled names or some missing parentheses, may be caught by a compiler, but many program deficiencies are not detected by compilers. For instance, a compiler will not catch the mistaken usage of x plus y, or the mistaken usage of x squared, in the place of x times y inside a program function that is meant to calculate an area based on x and y.
Compiled programs may be tested, e.g., by feeding them a set of inputs that should produce a certain known result, and comparing the program output to the desired result. Program inputs can be automatically generated, e.g., by scripts or fuzzer tools. But even with automation the set of possible inputs may be too large to test all of them as a practical matter. For example, it is not feasible to test an internet search engine by checking, for each possible input, whether correct search results are produced. There are too many possibilities, even at a frozen moment in time, and the set of possibilities continually changes. One may optimistically try to test a subset of inputs which is fully representative of all the possibilities, but choosing an input subset that will fully represent all possibilities can be difficult or impossible when the program's deficiencies are unknown, especially if some or all of the program source code is unavailable.
When source code is available, static analysis tools may find problems such as code that can never be reached regardless of the program input, code that may attempt division by zero, code that may cause a buffer overflow when executed, or code that depends on a variable which has not received a value, and so on. But even static analysis might not find every deficiency in a program.
In short, program testing and verification is a complex, challenging, and continually changing area of software development.